Using an unknown time zone with the from_utc_timestamp or to_utc_timetamp methods returns the time un-adjusted instead of throwing an error:
This should be an error because users may attempt to adjust to valid but unknown zones, like PDT or MDT. This would produce incorrect results with no warning or error.
Update: A good work-around is to add a table of known zones that maps to offset zone identifiers, like GMT-07:00. The table is small enough to always be a broadcast join and results can be filtered (e.g. offset_zone IS NOT NULL) so that only valid zones are passed to from_utc_timestamp.